Northwest Avalanche Center NWAC, Northwest Avalanche Center

Stevens Pass Avalanche Forecast

Jan 10th, 2020 10:00AM

The alpine rating is high, the treeline rating is considerable, and the below treeline rating is considerable. Known problems include Storm Slabs.

Avoid traveling in avalanche terrain above treeline, where very dangerous avalanche conditions can be expected. A deep winter storm cycle continues. Exercise cautious route finding to stay off of, and out from underneath steep slopes even well below treeline. Numerous very recent natural and human triggered avalanches have been observed, a key piece of information going into Saturday. 

Summary

Discussion

Heavy snow fell on Friday, and observers found very reactive storm slab instability on the new snow, old snow interface. Many soft storm slabs were observed on a range of aspects near and below treeline up to D2 (large). These were natural and human triggered, and on open slopes. Crowns were said to have "impressive propagation", and they were running "fast and far". We have minimal observations from the alpine, though multiple reports of moderate drifting from afar. The most recent crust was buried on January 7th, and extends up to about 5,000ft. Above that elevation it tapers away and all dry snow can be found. From January 6-7th, a major avalanche cycle occurred in the Stevens Pass corridor with lots of small, lots of large, and and a few very large (up to D3) in size. 

Small storm slab near Stevens Pass Mountain Resort. January 10, 2020. Josh Hirshberg photo

Snowpack Discussion

January 9th, 2020 (The regional synopsis is updated every Thursday @ 6 pm)

As we said Happy New Year and rang in 2020, snow was turning to rain at many trailheads and lower elevation Passes, not exactly the fresh start winter recreationalists had in mind. The snowpack was already looking a little thin throughout the region, especially at lower elevations. Low snow in places like Snoqualmie Pass made backcountry travel difficult and hazardous.  NWAC’s snow depth climatology report was showing snow depths 25-64% of normal to kick off the start of 2020.

Things can change quickly in the Pacific Northwest and they did as we entered an extended storm cycle between January 2nd to January 8th. Strong winds, fluctuating temperatures, and heavy precipitation offered few breaks in the weather over this period limiting observations and hampering travel. Despite periods of rain at lower elevations, most areas saw several feet of new snow with big jumps in total snow depths as a westerly storm track strongly favored the West Slopes of the Cascades and the Olympics for the highest precipitation totals.

Location

Total Snow Depth (in) 1/2/20

Total Snow Depth (in) 1/8/20

Hurricane Ridge

31

51

Heather Meadows Mt Baker

55

95

Washington Pass

49

74

Stevens Pass

41

63

Snoqualmie Pass

22

33

Alpental mid-mountain

44

63

Crystal Mt Green Valley

40

66

Paradise Mt Rainier

54

105

White Pass Upper

43

69

Timberline

36

57

Mt Hood Meadows

36

53

 

We may have started with a shallow snowpack, but most locations increased their snowpack by 70% or more over this storm cycle!

During this extended and impressive storm cycle that included backcountry avalanche warnings, natural avalanches were reported in many areas Jan 6th-7th.

The Stevens Pass area was especially active over the period with over 100(!) avalanche observations made on the 6th and 7th. Professionals reported numerous avalanches in places that they hadn't previously observed avalanches and some paths avalanched multiple times in a 24 hour period. Observers reported a few very large (size D2.5-3) avalanches, originating at upper elevations with deeper crowns that likely formed from wind drifting. Topping off an active couple of days, warming temperatures lead to a widespread loose wet avalanche cycle.

The southern Washington Cascades, the Wentachee Mountains and Mt. Hood either saw less precipitation, warmer temperatures leading to more rain than snow, or some combination of the two and ended up with relatively less active avalanche conditions than areas further north. 

A large natural avalanche on Rock Mountain near Berne along Hwy 2 east of Stevens Pass that released Jan 6th or 7th. Photo: Josh Hirshberg 1/7/20

Many small storm slabs released in the Crystal backcountry 1/6-1/7. Pinwheels in the photo suggest loose wet avalanche activity occurred when temperatures rose above freezing and snow turned to rain.

Another active and colder weather pattern is on it’s way. Enjoy yourself out there and be sure to check the forecast before heading out. Remember, NWAC is a community-supported avalanche center and when you submit an observation you make the forecast better!

-Peter Moore

It’s getting deeper! Photo: Jeremy Allyn

Problems

Storm Slabs

An icon showing Storm Slabs

Likelihood

very likely

Expected Size

1 - 1

Observers were finding instabilities at the new snow, old snow interface as the day progressed on Friday with impressively wide crowns. With even more new snow, these slabs are growing deeper. The instabilities are likely to linger with cold temperatures preserving strong over weak layering. Be observant as you travel, step off into virgin snow once in a while, and look for shooting cracks as you travel. At higher elevations and on open slopes, the wind has been drifting the new snow into thicker slabs. Even if you are finding less reactivity at lower elevations, these are likely to be more reactive, deeper, and more dangerous as one increases elevation. If you find signs of instability, it is time to step it back and avoid open slopes 35 degrees and steeper. 

Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

 

You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.

 

Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.

Aspects:

All aspects.

Elevations:

All elevations.

Valid until: Jan 11th, 2020 10:00AM